Certain vehicles have downstream surfaces, at or near the vehicle's rear, that are not streamlined. Examples include: over-the-road trucks and truck trailers, vans and minivans, motor homes and travel trailers, and pickup trucks. In vehicles such as pickup trucks, one non-streamlined surface is near the vehicle's center. Certain vehicles, such as semi-trailer trucks, have gaps between otherwise streamlinable exterior surfaces along the vehicle's length. In the case of the semi-trailer truck, a gap is typically present between a tractor unit and a semi-trailer. Certain of the above vehicles also include rear doors or tail gates.
It is known that a significant amount of aerodynamic drag is created when a vehicle travels at velocities typical on a modern roadway. This is due, in large part, to areas of low pressure that are induced on rearward surfaces of the vehicle. The low pressure becomes more pronounced as airflow over the vehicle separates from the rearward surfaces of the vehicle. The phenomenon of airflow separation is also known in aircraft wing design and, in this case, causes the wing to stall.
Moving vehicles having blunt rear ends and surfaces facing mainly rearward are especially affected by airflow separation starting at an abrupt transition between side, top, and bottom surfaces and the near vertical rearward facing surfaces. The low pressure that the airflow separation induces is compounded by a relatively large area on which the low air pressure acts in comparison with more streamlined vehicles.
Moving vehicles having an interruption in otherwise streamlined, near streamlined, or streamlinable exterior surfaces along the length of the vehicle are subjected to increased aerodynamic drag created by turbulence as airflow over the vehicle crosses the interruption.
The low air pressure (partial vacuum) acting on the rear surfaces of a moving vehicle produces a drag force that resists forward motion of the vehicle. Increased turbulence caused by gaps and interruptions in otherwise streamlined or streamlinable exterior surfaces also produces a drag force that resists the forward motion of the vehicle. These drag forces are opposed by the vehicle's engine and require power that is typically produced by burning fuel. Any reduction in aerodynamic drag results in a reduction in fuel consumption.
In an era of diminishing crude oil reserves, potential limits on atmospheric green house gas emissions in response to global warming, and increasing environmental consciousness, fuel efficiency improvements are a growing concern. Aerodynamic improvements are especially valuable since they can be combined with other improvements such as engine efficiency and reduced chassis weight. Increased fuel efficiency also provides a valuable benefit of increasing a range a given vehicle can travel before refueling.
At a given speed, streamline vehicles, such as certain high-speed cars and airplanes, have markedly lower airflow separation and therefore markedly lower aerodynamic drag in comparison with vehicles having blunt rear ends. Such streamline vehicles generally have gently tapering reward surfaces designed to keep airflow attached. A similar streamlining technique has also been applied to over-the-road trucks where it is known as a “boat-tail”. Boat-tails significantly increase the length of a vehicle. In addition, they may interfere with convenient access to the rear doors of the vehicle. There is a need for a device that provides streamlined rear surfaces to the rear of vehicles having blunt rear ends without introducing unacceptable limitations to the vehicle length or the rear door function. There is also a need to bridge gaps across discontinuities along the length of vehicles, such as the gap between the tractor unit and the semi-trailer of the semi-trailer truck. The present disclosure satisfies these and other needs.